1. Field of the Invention
This invention relates to apparatus for manufacturing blown tubular plastic film and in particular in apparatus for controlling the diameter, and therefore in the layflat width, of the blown tube by controlling the volume of cooling gas admitted to and exhausted from the blown tube.
2. Description of the Prior Art
The manufacturing of plastic film by the blown film process is well known in the art and reference may be had to U.S. Pat. No. 2,461,975 for a disclosure of the basic process. In this process as shown by the patent the diameter of the blown tube is controlled by pressure and volume of the trapped gas (usually air) which is introduced into the interior of the tube. This patent also discloses the use of a coolant gas impinging on the exterior of the tube to cool the plastic material after it leaves the die. In order to obtain high production rates it has been found desirable to provide additional cooling means which act on the interior of the tube, usually in conjunction with the aforementioned exterior means, as disclosed in French Pat. No. 1,190,773 and West German Pat. No. 1,106,951. The use of internal cooling according to these means requires that the cooling gas be continuously introduced and continuously withdrawn from the blown tube, in contrast to the trapped air as disclosed in U.S. Pat. No. 2,461,975. In order to control the diameter of the blown tube (and the consequent width of the layflat film) it is necessary to very precisely control the gas flow into and out of the tube. It is also desirable that these flows be controlled automatically, such as by means which continuously monitor the blown tube diameter and provide signals to flow control means to maintain the tube diameter within commericially acceptable limits.
Apparatus is known to monitor or sense the bubble diameter in the blown film process by means of electrical, mechanical or pneumatic devices. For example see British Pat. No. 1,017,398 and U.S. Pat. Nos. 3,125,616, 3,700,370 and 3,990,824. It is also known to provide damper means in the inlet and outlet gas ducts to control the gas flows, as disclosed in U.S. Pat. Nos. 3,762,853 and 3,898,028 among others.
In most cases, for example as when low density polyethylene is processed, the gas pressure in the blown tube is very low, only slightly above the atmospheric pressure on the exterior of the tube, while the gas flow is comparatively high to obtain adequate cooling. Control of the flow by a damper or dampers therefore must be very sensitive to variations in tube diameter and must adjust the comparatively high flow in very small increments.
Applicant has found that the relatively coarse control provided by the conventional damper in the gas flow duct or ducts makes it very difficult to obtain satisfactory control of the flow and has therefore conceived a novel flow control means that provides excellent control of the flow and yet is economical to manufacture and use.